Sioux City WWTP to upgrade biogas to renewable fuel

Earlier this year in Iowa, Sioux City Council members unanimously approved a project that will allow the city to capture, clean and compress gas at its wastewater treatment plant (WWTP), then sell it as renewable natural gas (RNG). The council voted 4-0 on Jan. 9 to award West Des Moines-based engineering firm Bartlett & West $1.29 million to perform all services leading up to construction on the project, as well as the engineering construction administration and observation services necessary during construction, startup and commissioning.

“Bartlett & West has been a key ally during the development of this concept and will act as our design consultant going forward,” said Mark Simms, Sioux City’s utilities director.

Since the plant’s original construction in 1961, anaerobic digestion (AD) technology has served as an integral component of the facility to reliably stabilize biosolids and produce biogas. The facility consists of eight digesters, two sets of four in the East and West complexes, which consist of two primary digesters and two secondary in each set. According to Simms, two of the digesters are not currently operating, but the intention is to eventually convert the two unused into primary digesters in order to maximize biogas production.

Until this most recent announcement, the last upgrade the plant received was in 2008 when measures to improve mixing and digester covers were put in place. Currently, some of the digested sludge is brought to the landfill or is used for land application. According to Simms, the city just recently entered into an agreement with a local land applicator and is now applying digested biosolids to fields that are owned by Sioux City as well as other farm fields in the area that can benefit from the application.

Methane gas is also used within the WWTP as a fuel for building heat and to heat the primary digesters. “The plant currently uses as much of the biogas as possible,” Simms said. “In the winter, the building heat and primary digester heating demands represent a relatively small fraction of the biogas produced, and the remainder is currently flared. They flare even more during summer months when heating of the facilities isn’t necessary.”

According to Philip Gates, PE with Bartlett & West, they plan to make all of the RNG available for vehicle fueling and will buy natural gas from the local utility as needed in order to maximize the generation of environmental credits. “At this time, we believe it is important to connect to the existing natural gas pipeline infrastructure,” Gates said. “This allows us the flexibility to contract anywhere in the country and to seek the best terms available for the city.”

He added that it is not necessary to use the gas locally but it certainly is possible for local fleets to take advantage in the future if they are interested in converting those fleets to compressed natural gas or liquefied natural gas. Upon completion, this project will produce the equivalent of 3.97 gallons of gasoline per minute, which amounts to just under 2.1 million gallons of gasoline offset in a year.

As far as the gas upgrading technology, Bartlett & West is completing 30 percent of the design now and, at the completion of that phase, will issue a request for proposals to gas purification vendors. “Our intent is to select and procure the purification equipment this summer,” Gates said. “This will allow Bartlett & West to design around the specific piece of equipment and allow the equipment to be manufactured while design is completed, thereby compressing the project schedule.”

The project is estimated to take about two years to complete, and the city anticipates advertising for bids in the first quarter of 2018.

With this project, Sioux City will be able to participate in the renewable fuel standard program, which was amended in 2014 to include RNG produced from biogas generated at a WWTP. “We are currently anticipating that the project will pay for itself within three years,” Gates said.

The renewable fuel portion of the project is estimated to cost $9.3 million, and it’s a part of a larger $25 million investment in upgrades over the next five years that will increase the capacity of the plant. Also on Jan. 9, the city council approved an agreement to award Bartlett & West an additional $605,000 for services related to this larger improvement project.

“The intent is for the revenue that is generated by the renewable fuels project to assist in keeping rate increases modest as we make the improvements needed to increase the plant capacity,” Simms said. “The increase in capacity is necessary for the city to accommodate additional industrial growth. This is a tremendous community for industries to locate and it is important for us to be able to provide the services required for industries to thrive here.”

According to Simms, the existing facility has a few bottlenecks that limit capacity, and these projects are designed to improve upon those. “We believe that the existing infrastructure will continue to serve the regional community well as the community grows, because the key unit processes could be cost effectively upgraded to provide service at significantly higher flow and loading conditions than we are currently targeting, in the targeted expansion, if necessary,” he said. “If significant growth is experienced beyond current expectations, the existing facilities can be tailored to handle it.”

The plant capacity will increase from around 16 million gallons per day to about 17 million gallons on an annual average daily flow capacity. The facility's biochemical oxygen demand rating will also increase by 18 percent. “The additional flow and organic loading capacity are needed to meet the growth anticipated over the planning period,” Simms said. “The city has completed an anti-degradation application and has been approved to receive up to 103,000 pounds of biochemical oxygen demand per day.”

He added that the projects identified for the WWTP will increase the plant’s rated capacity to simultaneously match the currently anticipated growth demands as well as the already approved waste load allocation utilized in the previously conducted anti-degradation review.

Simms said it’s possible that there will be subsequent phases to the renewable fuels project, but for now, there are only multiple phases planned for the other WWTP improvements. The first phase of the other improvements will include a $12.7 million asset renewal project designed to restore existing facilities, and a subsequent project will target the remainder of the improvements necessary to expand the capacity of the plant. “Depending on the rate of growth observed in coming years, it is possible that more than one plant expansion project may be necessary,” Simms said.

The Seaboard Triumph Foods facility will consume most of the existing, remaining capacity at the plant, and then, if the city wishes to continue to provide service to additional businesses or residents, Simms said, “it is vitally important that we provide the facilities necessary to allow it to be given additional rated capacity in order to keep progressing as an attractive location for businesses to locate.”